High dynamic range measurement system for process monitoring

ABSTRACT

A digital flashlamp controller, a flashlamp control system and a method of controlling a flashlamp bulb employing digital control electronics are provided herein. In one embodiment, the digital flashlamp controller includes: (1) a trigger interface configured to provide firing signals to control a trigger element for a flashlamp bulb and (2) digital electronics configured to generate the firing signals and control multiple pulsing of the flashlamp bulb.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/696,370, entitled “HIGH DYNAMIC RANGE MEASUREMENT SYSTEM FOR PROCESSMONITORING”, by Larry Arlos Bullock, et al., filed on Apr. 24, 2015,which is commonly assigned with the present application and isincorporated herein by reference as if reproduced herein in itsentirety.

TECHNICAL FIELD

The present invention relates to optical measurement systems and methodsof use. More particularly, the present invention is directed to anoptical measurement system configured to decrease sources of variationand extend dynamic range of measurement capabilities in aflashlamp-based optical measurement system.

BACKGROUND

Optical measurement systems are employed in a variety of industries,such as the semiconductor processing industry, for real-time monitoringof wafer modification and process control. Optical measurement systemsmay be integrated with a semiconductor processing tool and may beutilized in-situ for real-time process control or in-line for run-to-runfeedback control. Typically, monitored processes include semiconductoretching, deposition, implantation and chemical mechanicalplanararization processes for film thickness and plasma monitoringapplications.

Especially in the semiconductor processing industry, the use ofincreasingly variable material layers and features sizes(thinner/thicker layers, high aspect ratio features, very smallfeatures, mixed size features, highly variable reflectivity/absorptionmaterials, and high layer count stacks) has led to difficulties inachieving necessary levels of measurement accuracy and precision. Inaddition to the increasing complexity of the semiconductors themselves,highly integrated single chamber multiple step processes and dynamicprocessing tool changes of mechanical parameters (e.g., aperture andworking distances) cause variation in optical signal levels adverselyaffecting measurement accuracy and precision.

FIG. 1 shows a pictorial schematic of a typical prior art opticalmeasurement system 100. Optical measurement system 100 includes lightanalyzing device 110, light source 120, optical assembly 130, opticalfiber assembly 140, computer 150 and wafer 160. Light analyzing device110 is commonly a spectrograph, spectrometer, monochromator or otherlight analyzing device providing wavelength discrimination. Light source120 is either a continuous broadband emission source (e.g., tungstenhalogen lamp or deuterium lamp) or a pulsed broadband emission sourcesuch as a xenon flashlamp. Optionally, narrowband continuous or pulsedemission sources such as lasers and/or light emitting diodes are used.Optical assembly 130 is designed to direct light of one or morewavelengths emitted from light source 120 onto wafer 160 which istypically a silicon semiconductor wafer, sapphire substrate or otherworkpiece. Optical assembly 130 commonly acts to either focus orcollimate light from light source 120 onto wafer 160. Optical fiberassembly 140 is commonly a bifurcated optical fiber assembly whichdirects light from light source 120 to wafer 160 via optical assembly130 and subsequently directs light collected upon reflection from wafer160 via optical assembly 130 to light analyzing device 110. Computer 150is used to control light analyzing device 110 and light source 120 andis also used to analyze data collected by light analyzing device 110.Computer 150 may also provide signals to control external systems suchas semiconductor processing tools (not shown).

Reflectometry in the form of interferometric endpointing is widely usedin the semiconductor industry for monitoring the state of a waferprocess within a semiconductor processing tool by using optical signalsreflected from a wafer being modified within the processing tool. Whileinterferometric endpointing techniques may vary with the particularapplication and process, typically the light emission intensities aremonitored at one or more predetermined wavelengths. Depending on theprocess, various algorithms may be employed for deriving trendparameters, often related to thicknesses of various layers or featuresof the wafer, from the light intensities that are useful in assessingthe state of the semiconductor process and the in-process wafer,detecting faults associated with the process, processing tool or otherequipment. Although commonly named “endpointing” and historicallyimplying the detection of the end of a process; interferometricendpointing has evolved to include monitoring and measurement during alltimes of a process cycle.

With specific regard to monitoring and evaluating the state of a waferwithin a processing tool, FIG. 2 illustrates a typical prior art process200 for employing interferometric endpointing to monitor and/or controlthe state of a workpiece within a plasma processing tool. The presentmethod is greatly simplified for expedience. Details of certainprocesses and implementations are provided by review of US PatentApplication Number 20130016343, included herein by reference. Process200 typically begins by directing light onto the workpiece of interest(step 210). Light directed onto the workpiece is then reflected fromthat workpiece (step 220) and subsequently detected (step 230).Detection is commonly associated with conversion to electrical signals,the signals are typically amplified and then digitized and passed to asignal processor for analysis (step 240). The signal processor employsone of more algorithms that is/are specific to the particular productionprocess and the characteristics of the workpiece being monitored. Theselection of the proper algorithm, as well as parameter values, for theparticular process is imperative to achieving a valid result. Withoutbeing too specific, the algorithm analyzes intensity signals anddetermines trend parameters that relate to the state of the process andcan be used to access that state, for instance, end point detection,etch depth, film thickness, faults, plasma instability, etc. (step 250).The results are output (step 260) for use by external control systemsand/or engineers and then used for monitoring and/or modifying theproduction process occurring within the plasma processing tool (step270).

SUMMARY

In one aspect, the disclosure provides a digital flashlamp controller.In one embodiment, the digital flashlamp controller includes: (1) atrigger interface configured to provide firing signals to control atrigger element for a flashlamp bulb and (2) digital electronicsconfigured to generate the firing signals and control multiple pulsingof the flashlamp bulb.

In another aspect, a flashlamp control system is disclosed. In oneembodiment, the flashlamp control system includes: (1) a high voltagepower supply, (2) digital control electronics for controlling a dutycycle, frequency and pulse width of the high voltage power supply, (3)at least one capacitor electrically connected to the high voltage powersupply, (4) a trigger element controlled by the digital controlelectronics, and (5) a flashlamp bulb electrically connected to the highvoltage power supply.

In yet another aspect, a method of controlling a flashlamp bulbemploying digital control electronics is disclosed. In one embodiment,the method includes: (1) determining a power supply drive frequency,pulse width and duty cycle for a capacitor based on an active dischargecapacitor value and configuration thereof, wherein the capacitor iselectrically connected between a high voltage power supply and theflashlamp bulb, (2) generating a plurality of current pulses at the highvoltage power supply by the digital control electronics based on thepower supply drive frequency, pulse width and duty cycle, and (3)charging the capacitor employing the plurality of current pulses fromthe high voltage power supply.

BRIEF DESCRIPTION

Reference is now made to the following descriptions taken in conjunctionwith the accompanying drawings, in which: The novel features believedcharacteristic of the present invention are set forth in the appendedclaims. The invention itself, however, as well as a preferred mode ofuse, further objectives and advantages thereof, will be best understoodby reference to the following detailed description of an illustrativeembodiment when read in conjunction with the accompanying drawingswherein:

FIG. 1 is a pictorial schematic of a prior art optical measurementsystem;

FIG. 2 illustrates a typical prior art process for employinginterferometric endpointing to monitor and/or control the state of awafer within a processing tool;

FIGS. 3A and 3B show a set of plots representing challenges inherent inmeasurement of the reflectivity of thin films and the differences inreflectivity detection desired for high precision semiconductorprocesses and addressable, in accordance with an exemplary embodiment ofthe present invention;

FIG. 4 shows a pictorial schematic of a semiconductor processing toolincluding an optical measurement system, in accordance with an exemplaryembodiment of the present invention;

FIGS. 5A, 5B and 5C are a set of three cross-sectional views ofexemplary construction of an improved optical assembly interfacing witha flashlamp configurable for use with an optical measurement system, inaccordance with an exemplary embodiment of the present invention;

FIGS. 6A, 6B and 6C show a set of plots comparing the performance of aprior art optical measurement system and a flashlamp and opticalassembly, in accordance with an exemplary embodiment of the presentinvention;

FIG. 7 is a simplified electrical schematic of an improved flashlampcontrol system configurable for use with an optical measurement system,in accordance with an exemplary embodiment of the present invention;

FIGS. 8A and 8B show a set of plots comparing the performance of a priorart optical measurement system and an optical measurement systemincorporating a flashlamp modified in accordance with an exemplaryembodiment of the present invention;

FIGS. 9A-9E show a set of plots detailing the construction of acomposite high dynamic range signal provided by the functionality of anoptical measurement system incorporating the flashlamp control system ofFIG. 7, in accordance with an exemplary embodiment of the presentinvention; and

FIG. 10 shows a plot of the performance of the addition of a spectralflattening filter configurable for use with an optical measurementsystem, in accordance with an exemplary embodiment of the presentinvention.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanyingdrawings that form a part hereof, and in which is shown by way ofillustration, specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized. It is also to beunderstood that structural, procedural and system changes may be madewithout departing from the spirit and scope of the present invention.The following description is, therefore, not to be taken in a limitingsense. For clarity of exposition, like features shown in theaccompanying drawings are indicated with like reference numerals andsimilar features as shown in alternate embodiments in the drawings areindicated with similar reference numerals. Other features of the presentinvention will be apparent from the accompanying drawings and from thefollowing detailed description. It is noted that, for purposes ofillustrative clarity, certain elements in the drawings may not be drawnto scale.

Prior art systems, such as optical measurement system 100, are subjectto multiple herein-mentioned variations and limitations and have limitedsuitability for high repeatability and high accuracy opticalmeasurements, which limits their functionality for currentstate-of-the-art in situ and/or inline applications. To overcome theshortcomings of prior art systems, the present invention generallyincludes a system and method for optical measurement which compensatesfor the deleterious effects of limited dynamic range and variable signalattenuation as well as compensating for other system drift andvariation. More specifically, the present invention addresses: (1)increasing the dynamic range of optical measurement systems, (2)improving the stability of optical measurement systems for betteroptical measurement resolution and accuracy, and (3) providing theseprevious aspects in a multi-channel way. Other advantages of the currentinvention will be described below in association with embodiments.

The adjustability of prior art optical measurement systems is commonlybased upon a limited set of operational parameters including manualchanges to flashlamp capacitors, programmable flashlamp voltage controland/or configurable spectrometer integration time. These prior artsystems and their restrictive operational parameters become insufficientas the complexity of monitored processes increases as driven by theneeds of state-of-the-art semiconductor processing equipment andintegrated processes. Furthermore, although the generic method discussedabove with regard to FIG. 2 is useful in monitoring/evaluating manydifferent processes; the increasing precision, accuracy and variabilityof processes demand constant increases in dynamic range and noisereduction whereby adding further complexity and requirements to opticalmeasurement systems and processes.

FIGS. 3A and 3B show a set of plots 300 and 350, respectively,representing the reflectivity of thin films and the differences inreflectivity detection required for high precision monitoring ofsemiconductor processes. The example here of blanket silicon dioxidefilms deposited upon a crystalline silicon substrate is a simplificationof often very complex multilayer patterned film stacks but isnonetheless used as a reference and test geometry especially withcommonly used “monitor wafers.” Plot 300 shows the theoretical absolutereflectivity versus wavelength for 300 and 305 Angstrom thick blanketsilicon dioxide films deposited upon a crystalline silicon substrate. Asplotted the reflectances are essentially indistinguishable except nearwavelengths of approximately 350 nanometers. Remarkably, this differencein film thickness is comparable to only two monolayers of crystallinesilicon, and larger than two monolayers of silicon dioxide. These verythin films and the strict requirements upon their extremely highaccuracy and precision of film thickness determination are required foradvanced applications such as 3D NAND memory and gate structures in highspeed processors. Required control levels for film thickness are oftenat the level of 1 Angstrom. Although precision and accuracy of thicknessmeasurement at this level has historically required the application ofspectroscopic ellipsometry; the novel optical measurement system of thepresent invention also provides this level of measurement precision andconsequent process control.

Plot 350 shows a difference spectrum for the 300 and 305 Angstrom thickfilms represented in plot 300 experimentally obtained from an opticalmeasurement system of the present invention. For 5 Angstrom precision,the signal to noise must be approximately greater than 200:1 and for 1Angstrom precision greater than 1000:1. Maximal reflectance differences,near 300 nanometer wavelength are less than 0.5% for 5 Angstrom and lessthan 0.1% for 1 Angstrom differences in film thickness. Although asingle static measurement is indicated here, repeatability acrossmultiple measurements, from multiple points upon the same wafer andacross multiple semiconductor processing tools is required and furthertightens the dynamic range, signal-to-noise, channel-to-channelvariation and other requirements. The limitations inherent in existingprior art flashlamp based optical metrology systems and specifically inthe implementation of the flashlamps themselves inhibit obtainingrequired repeatability, uniformity, precision and accuracy.

FIG. 4 shows a pictorial schematic 400 of an integration of asemiconductor processing tool and an optical measurement system of thepresent invention. The system depicted here may be contrasted to theprior system depicted in FIG. 1 to permit discussion and motivation forinterferometric endpointing innovation to accommodate the dynamic toolconditions, multiple point measurement uniformity,multiple-film-single-chamber processes, etc. Critically, it should benoted that prior art optical measurement systems typically implement asingle measurement channel. Similarly, associated processing toolscommonly accomplish only a single process step. Advances in processingtool flexibility and process methodologies for larger wafers now requireadditional configurability in optical metrology systems in order tosupport increased operational parameters. For example, processing toolsmay change the gap (plasma space) and multiple measurement locations maybe affected by mechanical and/or thermal variations driven by processingtool changes. Furthermore, continuous metrology may be required in placeof common prior art “endpointing” for process stop.

System 400 includes major components: processing chamber 410,spectrometer 420, flashlamp 450, measurement control system 460 and toolcontrol system 470. Processing chamber 410 encloses wafer 412 in plasmavolume 414. Flashlamp 450 provides light to one or more fibers 452(indicated by dashed lines) which guide the light toward optics 455which are used to define the optical beams 457 and the measurement spotsize upon the surface of wafer 412. As wafer sizes increase from 200 mmto 300 mm and soon to 450 mm, more locations for measurement are neededto characterize the non-uniformity of the monitored process. Thisrequires simultaneous control over the various optical signal levels inorder to assure uniformity across the measurement locations. All opticalsignal levels must be precisely controlled for proper signal-to-noise,enabling the desired accuracy and precision of the resultantcharacterization, for example, film thickness determination.

Upon reflection from wafer 412 optical signals are transmitted viasignal optical fibers 459 (indicated by solid lines) to spectrometer 420for wavelength discrimination and conversion to electrical and/ordigital signals. Spectrometer 420 may, for example, include a gratingfor wavelength discrimination and CCD or CMOS optical sensors forcollection of optical signals. Fibers 459 may vary in length and make-upto allow for accessing the various locations of wafer 412 and mayinclude attenuators to adjust each signal and accommodate overall signallevel differences due to length, processing tool impacts or otherfactors. Spectrometer 420 may send and receive commands and otherinformation to and from flashlamp 450 via interconnect 422. Thesecommands may include lamp power level, flash rates, errors, multiplepulsing configurations, etc. as discussed herein. Spectrometer 420 mayalso communicate with measurement control system 460, commonly anindustrial PC via communication link 425 which may be use Ethernet, USB,EtherCAT or other systems and protocols. Alternatively measurementcontrol system 460 may be embedded into spectrometer 420 as a unifiedassembly. Spectrometer and/or measurement control system 460 maycommunicate via link 465 with tool control system 470 which itselfcommunicates/controls via link 472 with processing chamber 410 and otherelements (not shown) of the processing tool and semiconductormanufacturing facility.

Flashlamps are advantageous components of optical measurement systemsdue to their pulsed operation, brightness, controllability and cost.However, flashlamps can be significant sources of temporal and spectralvariation. An aspect of the present invention is to address thosevariations inherent in flashlamps to extend their usability byincreasing homogenization, reducing shot-to-shot temporal and spectralvariability and improving channel-to-channel uniformity for multipointmeasurement systems. FIGS. 5A, 5B and 5C are a set of threecross-sectional views of exemplary construction of an improved opticalassembly interfacing with a flashlamp configurable for use with anoptical measurement system and flashlamp 450, such as described inassociation with FIG. 4. In multipoint measurement systems, again withreference to FIG. 4, the optical output of flashlamp may be mapped ontoa suitable multichannel distributor. In FIGS. 5A, 5B and 5C multichanneldistributor 510 provides 1×N distribution of the optical signal providedby flashlamp bulb 520 and integrates the fiber connections to opticalfibers such as fibers 452 of FIG. 4. Although FIGS. 5A, 5B and 5C showspecific flashlamp bulb 520, and multichannel distributor 510 including7 fiber with SMA connection; it should be understood that greater orfewer fibers are possible and that different flashlamp bulbs and fiberconnections may be utilized.

Historically, for highest signal coupling common end of distributor 510is positioned as close as possible to face of flashlamp bulb 520.Although this may provide the highest overall signal couplingefficiency; the relative size of fiber bundle 515 and lamp arc 518 maycontribute to high channel-to-channel non-uniformity (see FIG. 6A systemconfigurations #1 and #2) due to angular and geometric size mismatch.For example, a 400 micron core fiber designed into a hexagonal closedpacked array as shown in bundle 515 has a diameter of approximately 1.35mm; whereas the spatial extent of the lamp arc 518 is an approximately1×2 mm flat-topped rectangular Gaussian profile. Furthermore the angularemission profile of the flashlamp bulb and the numerical aperture of theindividual fibers of bundle 515 are often incongruous. Partialoptimization may be achieved by selectively altering distance D1 betweendistributor 510 and lamp 520. This partial optimization comes at theexpense of signal level. Signal level may be adjusted by increasing lampoutput but this promotes a shorter lifetime of the lamp and isultimately undesirable. Experimentation indicates that distance D1 maybe set within the range of approximately 0.0 to 0.5 inches and morepreferably to a range of 0.2 to 0.3 inches to achieve improvedhomogenization and channel-to-channel non-uniformity.

FIG. 5B shows the inclusion of a first type of homogenizing element 530.Specifically element 530 may be, for example, a single- or double-sidedground glass diffuser, fly-eye lens assembly, holographic diffuser,microlens array or other imaging or non-imaging homogenizer includingvolume homogenizers. Homogenizing element 530 also provides partialhomogenization at the cost of signal level since considerable light maybe scattered and significant distances may be required to permit element530 to function properly. Experimentation indicates that for groundglass diffusers, such as from Thorlabs of Newton, N.J. gaps 535 and 538may be set within the range of approximately 0.0 to 0.5 inches and morepreferably to a range of 0.2 to 0.3 inches for gap 535 and 0.0 for gap538 respectively to achieve improved homogenization andchannel-to-channel non-uniformity. The application of homogenizingelement and appropriate gaps may be readily applicable in cases whereexcessive light is available since it is advantageous to operate thelamp at mid-range power levels for best performance and thereforeefficiency may not be critical.

FIG. 5C shows a second class of homogenizing element, light pipe 540,integrated between distributor 510 and bulb 520. Light pipe 540functions differently than homogenizing element 530 or inclusion of gapD1 and provides homogenization with limited loss of signal level. Forproper functioning of light pipe 540, gaps 545 and 548 must be carefullycontrolled and coaxial alignment of bulb 520, light pipe 540 anddistributor 510 must be maintained for proper filling of light pipe 540and interception of the homogenized signal onto distributor 510. In aspecific embodiment, gaps 545 and 548 may be set within the range ofapproximately 0.0 to 0.5 inches and more preferably to a range of 0.05to 0.25 inches for gap 545 and 0.0 for gap 548 respectively to achieveimproved homogenization and channel-to-channel non-uniformity for a 2 mmfused silica light pipe such as from Edmund Optics of Barrington, N.J.It should be noted that each method of homogenization discussed hereinabove in association with FIGS. 5A-5C provide improved homogenizationbut selective use conditions must be considered to balance the lightloss. Although transmissive optical elements are discussed hereinabove;it should be understood that reflective equivalents may exist and may besubstituted.

FIGS. 6A, 6B and 6C show a set of plots comparing the performance of aprior art system and a flashlamp and optical assembly in accordance withan exemplary embodiment of the present invention. Optical signalhomogenization directly affects both channel-to-channel uniformity andthe variation within each individual channel. Although thewavelength-dependent optical signal enters into the reflectance andultimate thickness measurement via complex trigonometric functions, thevariation in the reflectance and/or thickness measurement may beestimated as proportional to the variation in the optical signals. Theseplots show the improvements possible with the application of variedtypes of homogenization elements incorporated between the flashlamp bulband multichannel distributor as shown in FIGS. 5A-5C. Specifically, FIG.6A shows a plot 620 of the channel-to-channel uniformity and FIGS. 6Band 6C are within-channel improvements in coefficient of variation(“CV”) with and without referencing. The box and whisker chart of FIG.6A compares channel-to-channel uniformity amongst the prior artunhomogenized interfaces (configurations #1 and #2), diffusinghomogenizing elements (configurations #3 and 4) and homogenizing lightpipe elements (configurations #5 and #6). Values for plot 620 arederived from wavelength-averaged signals from a set of seven individualchannels. Configuration #3 represents a diffusing homogenizing elementwith smaller gaps for higher signal levels and results in poorerhomogenization and therefor lower channel-to-channel uniformity.Configuration #4 represents a diffusing homogenizing element with gapsoptimized for best uniformity. Configuration #5 represents a light pipehomogenizing element with a gap near the distributor slightly too largeand configuration #6 represents a light pipe homogenizing element withoptimized gaps. Experiments indicate that a light pipe homogenizingelement should be positioned so that it is essentially butted to thesurface of the flashlamp bulb and approximately one diameter (˜2 mm)from the distributor. It should be understood that light pipe diameterand position will depend upon factors such as bulb arc size, number anddiameter of the fibers in the distributor, etc.

FIG. 6B shows a plot of the within-channel and cross-channel performanceof a prior art seven channel system optical measurement system. For thissystem six channels are used for actual measurement and a seventhchannel is used for referencing the other channels for the mitigation ofsignal drift and other effects. In plot 640, the seven channels of dataare analyzed for variation individually (indicated by the leftmost 7bars) and then pairwise via wavelength by wavelength division forreferencing (indicated by individual bars of columns A-G). CV(coefficient of variation) values presented are averaged over thewavelength range of ˜300-700 nm. Individually the variation for eachchannel is ˜0.75% and the sixth channel is much worse at ˜1.5%.Considering the proportionality of the variation of the optical signalto the variation of resolved film thickness; it may be seen that the0.75% is insufficient to support the determination of the required 1Angstrom resolution discussed hereinabove which requires ˜0.1%variation. For convenience of operation, it is expected that all outputsfrom a distributor be equivalent but in the case of the prior art thisis not observed. Furthermore, when cross-channel variation is determined(columns A-G of plot 640), it is observed that correlation betweenchannels is sufficiently low that for all combinations the variation isactually increased so that although referencing may aid in themitigation of optical signal drift; the optical signal variation andtherefore the thickness resolution is negatively impacted.

In contrast to the data of FIG. 6B, the data of FIG. 6C shown in plot660 of the performance of a system of the present invention including anoptimized light pipe homogenizing element shows marked improvement. Allwithin-channel variation is greatly reduced and paired cross-channelvariation is similarly reduced and in some cases the correlation is nowsufficiently high to permit the referencing to further reduce thevariation as well as controlling drift.

FIG. 7 is a simplified electrical schematic of flashlamp control system700 configurable for use with an optical measurement system, inaccordance with an exemplary embodiment of the present invention. FIG. 7represents the primary elements required for operation of flashlampcontrol system 700. Commonly known and used secondary elements such assignal conditioning components, filter capacitors, DC-DC convertors,etc. are not displayed for clarity of the primary elements. Theschematic indicates the essential components of an improved dynamicallyadjustable, by internal and external digital electronic means, flashlampcontrol system. This is in contrast to fixed, static prior art systemssuch as shown in U.S. Pat. No. 3,780,344, included herein by reference.

Control system 700 includes configurable digital control electronics 710which provide interfacing to both internal and external components suchas spectrometer 420 of FIG. 4 and internal high voltage power supply720. Flashlamp control system 700 may also include any number of staticcapacitors 730 (only one is shown for simplicity) and any number ofdynamically electronically switchable capacitors 740 (only one is shownfor simplicity). It should be understood that parallel and serialcombinations of capacitors 730 and 740 are possible. Static capacitor730 may be serially connected with a current sensing component 735 whichmay be used to detect correct or faulty operation for charging anddischarge of the capacitor and providing feedback via digital controlelectronics 710 to internal and external systems for example to indicatea proper or improper lamp discharge. In accordance with other exemplaryembodiments of the present invention (not shown in FIG. 7), additionalcurrent sensing components may be serially connected with any additionalstatic or dynamically electronically switchable capacitor.

Any dynamically electronically switchable capacitor 740 may be isolatedfrom control system 700 by switching isolation switch 745 during theuncharged state of dynamic capacitor 740. Switch 745 may be, forexample, as described herein below. Flashlamp bulb 750 is connected in aparallel arrangement with capacitors 730 and 740. Firing of flashlampbulb 750 is initiated by a signal from digital control electronics 710activating triggering transistor 770 whereby discharging triggercapacitor 760 and causing flashlamp bulb 750 to fire.

Prior art references show a variety of different switch configurationsfor triggering a flashbulb, however, the control circuit to the switchis always analog. The incorporation of flashlamp digital controlelectronics, in accordance with exemplary embodiments of the presentinvention, improves flashlamp performance and functionality in fivecritical areas. It (1) decreases capacitor recharge time (system cycletime), (2) improves flash to flash intensity and spectral stability, (3)extends the usable dynamic range of the system, (4) permits dynamicchange of capacitors and lamp output, and (5) provides monitoring forlamp discharge fault conditions. By knowing the active dischargecapacitor value and configuration, digital control electronics 710calculates an optimum power supply drive frequency and duty cycle tocharge capacitors 730 and 740 as rapidly as possible within the systempower constraints. In the prior art, the typical side effect of rapidcharge time is high voltage ripple that results in poor flash intensityand spectral stability. The configurability of digital controlelectronics 710 prevents this side effect and minimizes high voltageripple by dynamically changing the drive frequency and duty cycle as thecharge voltage of capacitors 730 and 740 nears the designated dischargevoltage. The residual high voltage ripple effects from this advancedcontrol scheme are significantly lower than the intrinsic bulbinstability, removing high voltage power supply ripple as a cause offlashlamp instability. Although the system dynamic range is limited bythe stable voltage operating range of the lamp (typically ˜300V-1 kV forthis type of lamp), digital control electronics 710 is able todynamically switch capacitors to greatly expand the stable operatingdynamic range of the system while maintaining the lamp within itsvoltage specification. Since digital control electronics 710 activelymonitors system voltages and currents during operation, it can providefault notifications if abnormal operation of the bulb or system occurs.

Additional features of control system 700 include configuring theprimary side switch of high voltage power supply 720 as high power, highvoltage metal-oxide-semiconductor field-effect transistor (“MOSFET”)device for improved performance and control. Furthermore, the lamptriggering transistor 770 is configured as a high current, high voltageMOSFET device. This specific embodiment provides improved trigger pulsecontrol over traditional silicon-controlled rectifier (“SCR”) basedcircuits. The dynamic capacitor switching is accomplished by switch 745based upon a configuration of high voltage diodes and SCRs. The uniquetopology of switch 745 eliminates the distortion in the dischargecurrent waveform due to SCR turn-on times. The capacitors are nominallylow-side switched when discharged, so only the charging cycle currentsare switched, not affecting the discharge current waveform.

Digital control electronics 710 may be implemented by using afield-programmable-gate-array (“FPGA”) such as from Altera or Xilinx, amicrocontroller such as a “PIC” series devices from Microchip or anembedded microprocessor such as ARM series devices from Atmel. In onepossible embodiment digital control electronics 710 is implemented in aflash-based Xilinx FPGA. This specific implementation providesadditional benefits by eliminating boot and initialization issues/delayarising from a SRAM-based device. The configurability andfeedback/control capabilities provided by digital control electronics710 strongly improves upon the historical application of analog controlsystems.

FIG. 8A shows a plot 800 comparing the performance of a prior art systemand a flashlamp modified in accordance with an exemplary embodiment ofthe present invention. Plot 800 presents a comparison of thewavelength-averaged CV (coefficient of variation as percent of signalaveraged over 200-800 nm spectral range) for operation of a flashlampover three different modes of operation versus data interval. The dataare presented for operation at low power (capacitor charge voltages˜350-400V) where the deleterious effects of flashlamp operation are mostprevalent. Curve 805 is a reference set of data indicating theperformance of prior art system with fixed charging rates and statichigh voltage power supply conditions. Poor performance of the chargingcircuit is clearly indicated as it results in overall high CV andconsiderable structure in curve 805 as the data interval is increasedthereby changing is interacting with the recharge rate. The strongstructure in curve 805 is due to variations in the charge state of thecapacitor varying between each firing in some cases being charged morethan expected and in other being charged less than expected. This isspecifically notable for ˜25 and 45 millisecond recharge periods. Curve810 shows the markedly improved performance achieved with implementationof the flashlamp control system of FIG. 7. Overall there is a 4×-6×improvement in the CV to approximately 0.25%. This supports theextremely low noise requirements for the high precision and accuracymeasurements discussed previously herein in association with FIG. 3.Additionally, curve 815 indicates the additional performance benefitsprovided by multiple pulsing, in this case 2×, of the flashlamp (enabledby the improvements in control system 700 including digital controlelectronics 710).

FIG. 8B shows chart 840 of the extended dynamic range provided by thecapacitor switching and multiple pulsing improvements of flashlampcontrol system 700. In chart 840 the log base 10 of the drive energy isplotted versus the configuration of the capacitors. The drive energy isused here as it is proportional to the optical output by a determinableefficiency factor. In the example shown, static small capacitor C1 of0.033 μF is included with dynamically switchable large capacitor C2 of0.15 μF. Bar 843 indicates the range of drive energy for capacitor C1over the drive voltage range 350 to 1000 V corresponding to driveenergies of 2 mJ and 16.5 mJ, respectively. Bar 845 indicates bothcapacitors C1 and C2 switched-on so as to drive the lamp again from withdrive charge voltage from 350 to 1000 corresponding to drive energies of11 mJ and 91.5 mJ, respectively. Bar 850 indicates both capacitors C1and C2 switched-on so as to drive the lamp with 2× multiple pulsing upto 183 mJ of drive energy. Typical flashlamp operation is limited to avoltage range of 350 to 1000 volts due to operational characteristicsfor proper excitation of the flashlamp bulb and the drive energy as wellas the optical output scales as E=0.5*C*V*V. These constraints limit therange of output of traditional systems to from 12.25% to 100% (˜8×range) regardless of capacitor size. As may be readily seen, theimprovements for dynamic switching and multiple pulsing capabilitiesprovided by the advanced flashlamp control system easily permits outputranges in excess of 90× (183 mJ/2 mJ) with the inclusion of a singleswitchable capacitor without adverse impact. It should be noted that forprior art fixed flashlamp charging circuits using capacitors of thelarge ratio as in the described example would result in a compromisedsolution with poor performance of the small capacitor and/or slowcharging times of the large capacitor.

FIGS. 9A-9E show a set of plots detailing the construction of acomposite high dynamic range signal provided by the multiple pulsingfunctionality of the flashlamp control subsystem of FIG. 7 synchronizedwith optical signal measurement via spectrometer, in accordance with anexemplary embodiment of the present invention. Multiple pulsing permitseffective changes in the output signal level of the flashlamp systemwhile maintaining steady state performance and temperature of a fixedenergy per flash periodic discharge. Since, as discussed above,shot-to-shot variation is very small and since variable drive voltagesare not used each output is nearly identical; combining differentlymultiply pulsed signals in cooperation with a spectrometer andmeasurement control system results in a composite high dynamic rangesignal with decreased CV. FIG. 9A shows plot 905 of exemplary signals907 and 910 from flash multipliers of 1× and 4× pulsing, respectively.FIG. 9B shows plot 915 of the CVs 917 and 920 of exemplary signals 907and 910 from the 1× and 4× pulsing, respectively. Although the signaland CV from the 4× multiply pulsed signal is improved over the CV fromthe 1× pulsing case; certain portions of the original signal aresaturated and corresponding portions of the CV are undefined andtherefore as a whole the spectrum is not useful for measurement.However, corresponding portions of signal 907 and CV 917 are madeavailable by the synchronization between spectrometer and flashlamp andinternal flashlamp control.

FIG. 9C shows plot 925 of a spectral mask used to combine spectra 907and 910 into a new lower CV and higher dynamic range signal shown inFIG. 9D. Solid curve 927 is the wavelength dependent mask value andshaded portions of plot 925 indicate spectral regions where the multiplypulsed signal 910 is saturated. To use the mask, spectrum 907 ismultiplied by the mask and spectrum 910 is multiplied by the additivecomplement value (1−mask) of the mask. FIG. 9D shows plot 930 of theresultant higher dynamic range spectrum 935 and FIG. 9E shows plot 940of the resultant CV 945 for the spliced spectra. Comparing FIG. 9E toFIG. 9B an improvement of approximately 2× as indicated by decreased CVmay be observed.

FIG. 10 shows plot 1000 of the performance of the addition of a spectralflattening filter configurable for use with an optical measurementsystem, in accordance with an exemplary embodiment of the presentinvention. As seen by comparison of unflattened signal 1010 andflattened signal 1020, spectral flattening typically severely decreasessignal levels. The complex spectral structure of the output of a xenonflashlamp is particularly difficult to flatten and results in a decreaseof approximately 10× in signal level at certain wavelengths. Although,this very significant amount of signal is lost by transmission throughthe filter, the flattened signal provides and equivalent increase in thedynamic range of an optical measurement system at many wavelengths bymore uniform matching the spectral signal to the detection capabilitiesof the optical measurement device such as a spectrometer. The multiplepulsing capabilities provided by the flashlamp control system of FIG. 7may be advantageously employed to recover useful signal levels withoutrequiring excessive drive powers for flashlamps.

The changes described above, and others, may be made in the opticalmeasurement systems and subsystems described herein without departingfrom the scope hereof. For example, although certain examples aredescribed in association with semiconductor wafer processing equipment,it may be understood that the optical measurement systems describedherein may be adapted to other types of processing equipment such asroll-to-roll thin film processing, solar cell fabrication or anyapplication where high precision optical measurement may be required.Furthermore, although certain embodiments discussed herein describe theuse of a common light analyzing device, such as an imaging spectrograph;it should be understood that multiple light analyzing devices with knownrelative sensitivity may be utilized. Furthermore, although the term“wafer” has been used herein when describing aspects of the currentinvention; it should be understood that other types of workpieces suchas quartz plates, phase shift masks, LED substrates and othernon-semiconductor processing related substrates and workpieces includingsolid, gaseous and liquid workpieces may be used.

The exemplary embodiments described herein were selected and describedin order to best explain the principles of the invention and thepractical application, and to enable others of ordinary skill in the artto understand the invention for various embodiments with variousmodifications as are suited to the particular use contemplated. Theparticular embodiments described herein are in no way intended to limitthe scope of the present invention as it may be practiced in a varietyof variations and environments without departing from the scope andintent of the invention. Thus, the present invention is not intended tobe limited to the embodiment shown, but is to be accorded the widestscope consistent with the principles and features described herein.

The flowchart and block diagrams in the figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems which perform the specified functions or acts, or combinationsof special purpose hardware and computer instructions.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

As will be appreciated by one of skill in the art, the present inventionmay be embodied as a method, system, or computer program product.Accordingly, the present invention may take the form of an entirelyhardware embodiment, an entirely software embodiment (includingfirmware, resident software, micro-code, etc.) or an embodimentcombining software and hardware aspects all generally referred to hereinas a “circuit” or “module.” Furthermore, the present invention may takethe form of a computer program product on a computer-usable storagemedium having computer-usable program code embodied in the medium.

Those skilled in the art to which this application relates willappreciate that other and further additions, deletions, substitutionsand modifications may be made to the described embodiments.

What is claimed is:
 1. A flashlamp control system comprising: a highvoltage power supply; at least one capacitor electrically charged bysaid high voltage power supply, said at least one capacitor having acharge voltage and a designated discharge voltage; digital controlelectronics for dynamically changing a drive frequency and duty cycle ofsaid high voltage power supply as said charge voltage of said at leastone capacitor approaches said designated discharge voltage within lessthan a kilovolt difference between said charge voltage and saiddesignated discharge voltage; a trigger element controlled by saiddigital control electronics; and a flashlamp bulb electrically connectedto said high voltage power supply.
 2. The flashlamp control system asrecited in claim 1 wherein said digital control electronics includes asensing interface configured to receive a feedback control signal thatcorresponds to a charging or discharging current of said at least onecapacitor.
 3. The flashlamp control system as recited in claim 2 whereinsaid digital control electronics is configured to determine a drivefrequency, a pulse width and a duty cycle for said high voltage powersupply based on said feedback control signal.
 4. The flashlamp controlsystem as recited in claim 1 wherein said digital control electronics isfurther configured to determine a discharge fault of said flashlampbulb.
 5. The flashlamp control system as recited in claim 1 wherein saiddigital electronics is further configured to actively monitor operationof said flashlamp bulb.
 6. The flashlamp control system of claim 1,wherein said digital control electronics controls a recharge rate, anenergy level setting and multiple pulsing of said flashlamp bulb.
 7. Theflashlamp control system of claim 1, wherein said at least one capacitoris a dynamically switchable capacitor and said flashlamp control systemfurther includes an isolation switch electrically connected to saiddynamically switchable capacitor and said digital control electronics,wherein said digital control electronics controls operation of saidisolation switch.
 8. The flashlamp control system as recited in claim 7wherein said dynamically switchable capacitor is coupled in parallelwith said flashlamp bulb.
 9. The flashlamp control system of claim 7,wherein said digital control electronics identifies an uncharged stateof said dynamically switchable capacitor and sends a switch signal tosaid isolation switch for electrically switching said isolation switchthereby isolating said dynamically switchable capacitor during theuncharged state of said dynamically switchable capacitor.
 10. Theflashlamp control system of claim 9, wherein when said at least onedynamically switchable capacitor is discharged, only charging cyclecurrents are switched to the isolated dynamically switchable capacitor.11. The flashlamp control system of claim 10, further comprising: acurrent sensing component electrically connected to said dynamicallyswitchable capacitor for monitoring at least one of charge current anddischarge current of said dynamically switchable capacitor, said currentsensing component further electrically connected to said digital controlelectronics for sensing the monitored at least one of charge current anddischarge current of said dynamically switchable capacitor.
 12. Theflashlamp control system of claim 1, further comprising: a homogenizingelement positioned proximate said flashlamp bulb for modifying saidoptical signal by decreasing the coefficient of variation of saidoptical signal both temporally and spectrally.
 13. The flashlamp controlsystem of claim 12, wherein said homogenizing element is selected fromthe group consisting of predetermined air gaps, diffusing homogenizingelements, imaging elements, non-imaging elements and light pipehomogenizing elements.
 14. The flashlamp control system of claim 12,wherein the coefficient of variation of said modified optical signal is0.25% or less and the coefficient of variation between each output is10% or less.
 15. The flashlamp control system of claim 1, furthercomprising: a spectral flattening filter positioned proximate saidflashlamp bulb for modifying said optical signal by decreasing thespectral intensity variation of said optical signal; a spectral mask formasking first wavelength values in a first spectra obtained at a firstpredetermined flash multiplier or energy; and a spectral splicer forcombining said first spectra with said masked first wavelength valueswith a second spectra obtained at a second predetermined flashmultiplier or energy using said spectral mask to construct a highdynamic range spectral signal.
 16. A method of controlling a flashlampbulb employing digital control electronics, comprising: dynamicallychanging a drive frequency and duty cycle of a high voltage power supplyas a charge voltage of a capacitor, electrically connected between saidhigh voltage power supply and said flashlamp bulb, approaches adesignated discharge voltage of said capacitor within less than akilovolt difference between said charge voltage and said designateddischarge voltage; generating a plurality of current pulses at said highvoltage power supply by said digital control electronics based on saidpower supply drive frequency and duty cycle; and charging said capacitoremploying said plurality of current pulses from said high voltage powersupply.
 17. The method of claim 16 further comprising controlling, bysaid digital control electronics, a recharge rate, an energy levelsetting and multiple pulsing of said flashlamp bulb.